Ankündigung des Promotionsvortrags von: Herrn Alexander Tafel
Many modern techniques like ultrafast electron microscopy, dielectric laser acceleration and ultrafast electron diffraction require electron beams with high beam quality, short electron pulse duration and high bunch charges. However, in most state-of-the-art experimental setups, suboptimal electron sources are used: flat photocathodes with comparably high beam emittance or electron sources which were designed for continuous emission.
For this reason, we designed, fabricated and characterized a novel high brightness femtosecond-laser-induced photoemitter: Nanodiamond-coated tungsten needle tips.
We report on a reliable fabrication recipe which allows conformal coating of tungsten needle tips with sub-100 nm thin nanocrystalline diamond.
Via electron energy loss spectroscopy and electron diffraction, we confirm the presence of diamond and graphitic phases and evaluate the sp2 to sp3 ratio with nanometer resolution. We find graphitic boundaries between the diamond grains and an elevated sp2 content at the emitter apex.
Femtosecond laser-induced photoemission from these samples reveals a complex and fascinating emission behavior. Electrons can be photoexcited for wavelengths from the infrared (1932 nm) to the ultraviolet (235 nm) because multiphoton excitation becomes efficient over the entire spectral range.
Depending on the laser wavelength, we find different dominant emission channels identified by the number of photons needed to emit electrons. Based on the band alignment between tungsten and nanodiamond, the relevant emission channels can be identified as specific transitions in diamond and its graphitic boundaries.
Additionally, the emitter performance is promising for a future use in applications. Emission is stable for all wavelengths and bunch charges investigated. We infer a normalized emittance of < 0.2 nm rad and a normalized peak brightness of > 1.2 A m-2 sr-1.
(Vortrag auf Englisch)
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